In order to form a large-diameter synthetic-resin tube suitable for conversion into a succession of bags or the like, it is necessary to transform a small-diameter solid strand of a thermoplastic resin such as polyethylene into a relatively large-diameter tube. This relatively large-diameter tube can be made even larger by inflation with a gas that also serves to cool and harden the resin.
This operation is normally done by a nozzle assembly having at one end a small-diameter inlet that receives the continuously extruded and solid synthetic-resin strand, and at its other end a large-diameter annular orifice from which the large-diameter tube issues. Between these two ends the nozzle is formed internally with an annular and elongated passage centered on an axis. In this passage there is provided a mixing formation, in the form of a so-called spiral or helical distributor, which ensures that the solid strand is mixed thoroughly to form a homogeneous tube.
In order to ensure that the tube issuing from the outlet is perfectly homogeneous, it is necessary normally to make the nozzle assembly axially relatively long. In this manner the resin flows along a relatively long path and in this path the gaps between portions of the resin are largely eliminated with good homogenization of the resin. The disadvantage of this system is plainly that such a nozzle assembly is expensive to manufacture and maintain due to its large size. Furthermore such a large size requires that the entire production unit also be relatively long.